Glass composition for crystallized glass

ABSTRACT

A polished glass disk medium substrate suitable for use as a substrate for a hard disk, a hard disk containing the substrate and methods for making the substrate. The substrate containing glass forming raw materials may be formed so as to have a Young&#39;s modulus of 110 or higher.

RELATED APPLICATION

[0001] This application claims priority to Japanese Patent ApplicationNo. 2000-100829 filed in Japan on Apr. 3, 2000, the contents of whichare hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a glass composition, andspecifically relates to a glass composition suited for crystallizedglass. More specifically, the present invention relates to a compositionfor crystallized glass disk medium. Such disk medium include hard disks,magnetic disks, optical disks and magnetic-optical disks

DESCRIPTION OF THE PRIOR ART

[0003] Aluminum and glass are known materials suitable for use asmagnetic disk substrates. Among these substrates, glass substrates havebeen the focus of most attention due to their superior surfacesmoothness and mechanical strength. Such glass substrates includechemically reinforced glass substrates strengthened by ion exchange onthe surface, and crystallized glass substrates having strengthened bondsby depositing a crystal component on the substrate.

[0004] The performance demands of recent substrates have become moresevere day by day, and improved performance is particularly soughtregarding strength, flex and warp during high-speed rotation. This typeof performance can be expressed by the Young's modulus of the substratematerial, in which a higher numerical value is desirable.

[0005] For example, the composition disclosed in Japanese Laid -OpenPatent Application No. 11-322362 attains a Young's modulus value of 130or greater. However, this prior art requires extremely high thermalprocessing temperatures which complicate the manufacturing process, thatis, this art requires a primary processing temperature of 800° C., and asecondary processing temperature of 1,000° C.

SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide an improvedglass composition.

[0007] Another object of the present invention is to provide a glasscomposition having a high Young's modulus and which is highly suited formass production.

[0008] These objects are attained with a glass composition of thepresent invention desirably having the main components within the rangesdescribed below:

[0009] about 35 wt % or more, but less than about 50 wt % SiO₂;

[0010] about 13 wt % or more, but less than about 15.5 wt % Al₂O₃;

[0011] about 10 wt % or more, but less than about 30 wt % MgO; and

[0012] about 5 wt % or more, but less than about 20 wt % TiO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] The preferred embodiments of the present invention are describedhereinafter.

[0014] These objects are attained with a glass composition of thepresent invention desirably having the main components within the rangesdescribed below:

[0015] about 35 wt % or more, but less than about 50 wt % SiO₂;

[0016] about 13 wt % or more, but less than about 15.5 wt % Al₂O₃;

[0017] about 10 wt % or more, but less than about 30 wt % MgO;

[0018] about 5 wt % or more, but less than about 20 wt % TiO₂.

[0019] When the composition content Of SiO₂ used as a glass formingoxide is less than about 35 wt %, melting characteristics are typicallyadversely affected, and when the percentage exceeds about 50 wt %, astabilized state of glass is achieved and crystal deposition typicallybecomes difficult.

[0020] Aluminum oxide (Al₂O₃) is an intermediate oxide of glass, and isa structural component of the crystal-phase magnesium-aluminum crystalsformed during heating. When the composition content is less than about13 wt %, there are typically few crystals formed, and the desiredstrength is not obtained, whereas when the percentage exceeds about 15.5wt %, the melting temperature is typically raised and devitrificationreadily occurs.

[0021] Magnesium oxide (MgO) is a fluxing agent, which is added toinduce the crystal particles to nucleate and form crystal particleclusters. When the composition content is less than about 10 wt %, theworking temperature range is typically narrowed, and the chemicaldurability of the glass matrix phase is not typically improved. When thecomposition content exceeds about 30 wt %, other crystal phase matter isoften deposited and the desired strength is typically difficult toobtain.

[0022] Titanium oxide (TiO₂) is a crystal nucleating agent, which isoften an essential component for magnesium silicate crystal deposition.Furthermore, TiO₂ functions as a fluxing agent to improve stabilityduring production. When the composition content is less than about 5 wt%, melting characteristics are typically adversely affected, and crystalgrowth is often difficult. When the content exceeds about 20 wt %,crystallization typically progresses rapidly, the crystallization stateoften becomes difficult to control, the deposited crystals are typicallycoarse with heterogeneity of the crystal phase, and a fine homogeneouscrystal structure often cannot be obtained, such that the requiredsurface smoothness for use as a disk substrate is difficult to obtain bya polishing process. Furthermore, devitrification readily occurs duringfusion molding, and mass production characteristics are reduced.

[0023] The manufacturing method is described below. The raw materials ofthe ultimately produced glass substrate are thoroughly mixed in specificproportions, then introduced to a platinum crucible and melted. Aftermelting, the melted material is poured into a mold to form anapproximate shape. Then the material is annealed to room temperature.Next, the material is maintained at a primary heating processtemperature of about 500 to about 680° C. during a primary process(heating process) to generate crystal nuclei. Then, the material ismaintained at a secondary heating process temperature of about 680 toabout 800° C. during a secondary process to grow crystal nuclei. Thenthe material is cooled to obtain the crystallized glass.

[0024] This material may be used as a disk substrate by processing suchas polishing to attain a desired shape and thickness.

[0025] By using the above raw materials and the process describedherein, an extremely high Young's modulus and high mass productioncharacteristics are obtainable. Even higher performance is obtained byadding the components described below in a suitable range.

[0026] Stability during manufacture is improved by the addition of Li₂O,which functions as a fluxing agent. When the composition content is lessthan about 0.1 wt %, there is inadequate improvement in meltingcharacteristics. When the composition content exceeds about 8 wt %,stability often decreases during the polishing and washing processes.

[0027] Zinc oxide (ZnO) functions as a fluxing agent which augmentsuniform crystal deposition. When the composition content is less thanabout 0. 1 wt %, there is typically insufficient improvement in crystalhomogeneity. When the composition content exceeds about 22 wt %, theglass becomes stable, and crystallization is suppressed, such that thedesired strength is often difficult to obtain.

[0028] Phosphoric anhydride (P₂O₅), which functions as a fluxing agent,is a nucleating agent for depositing silicate crystals, and is animportant component for uniform deposition of crystals on the entiretyof the glass. When the composition content is less than about 0.1 wt %,sufficient formation of crystal nuclei typically becomes difficult,crystal particles are often coarse, heterogeneous crystal depositionoften occurs, the desired fine homogeneous crystal structure may bedifficult to obtain, such that the required surface smoothness for useas a disk substrate may be difficult to obtain by a polishing process.When the content exceeds about 5.0 wt %, reactivity to the filter mediumincreases during melting, and devitrification increases so as to reducemass production characteristics during fusion molding. Chemicaldurability typically decreases, there is concern that the magnetic layermay be affected, and stability is often reduced during the polishing andwashing processes.

[0029] Adding ZrO₂ which functions as a glass modifying oxidant alsofunctions effectively as a glass crystal nucleating agent. When thecontent ratio is less than about 0.1 wt %, sufficient formation ofcrystal nuclei typically becomes difficult, crystal particles are oftencoarse, heterogeneous crystal deposition often occurs, the desired finehomogeneous crystal structure may be difficult to obtain, such that therequired surface smoothness for use as a disk substrate may be difficultto obtain by a polishing process. Furthermore, chemical durability andmigration resistance are often reduced, there is concern that themagnetic layer may be affected, and stability is often reduced duringthe polishing and washing processes. When the content exceeds about 12wt %, the melting temperature is raised, devitrification readily occurs,and fusion molding typically becomes difficult. Furthermore, thedeposition crystal phase fluctuates such that desired characteristicsare often difficult to obtain.

[0030] The addition of CaO, which functions as a fluxing agent,supplements uniform crystal deposition. When the composition content isless than about 0.1 wt %, sufficient improvement in crystal homogeneityis not typically obtained. When the content exceeds about 9 wt %,chemical durability is not typically improved.

[0031] Crystal nucleating material is increased by the addition ofNb₂O₅, which works as a fluxing agent. When the composition content isless than about 0.1 wt %, there is often inadequate rigidityimprovement. When the composition content exceeds about 9 wt %,crystallization of the glass typically becomes unstable, the depositioncrystal phase typically becomes uncontrollable, and the desiredcharacteristics are often difficult to obtain.

[0032] The addition of Ta₂O₅, which works as a fluxing agent, improvesfusion and strength, and also improves chemical durability in the glassmatrix phase. When the composition content is less than about 0.1 wt %,there is typically inadequate rigidity improvement. When the compositioncontent exceeds about 9 wt %, crystallization of the glass typicallybecomes unstable, the deposition crystal phase becomes uncontrollable,and the desired characteristics are often difficult to obtain.

[0033] Stability during manufacture is improved by the addition of K₂O,which functions as a fluxing agent. When the composition content is lessthan about 0.1 wt %, there is inadequate improvement in meltingcharacteristics. When the composition content exceeds about 9 wt %, theglass typically becomes stable and crystallization is suppressed,chemical durability is often reduced, and there is concern that themagnetic layer will be affected, and stability often decreases duringthe polishing and washing processes.

[0034] Glass phase splitting is promoted by adding B₂O₃, which works asa former, and accelerates crystal deposition and growth. When thecomposition content is less than about 0.1 wt %, improvement of meltingcharacteristics is typically inadequate. When the composition contentexceeds about 9 wt %, glass devitrification readily occurs, moldingtypically becomes difficult, and the crystals often become coarse suchthat fine crystals is difficult to obtain.

[0035] Rigidity is improved by adding Y₂O₃, which functions as a fluxingagent. When the composition content is less than about 0.1 wt %, thereis typically inadequate rigidity improvement. When the compositioncontent exceeds about 9 wt %, crystal deposition is often suppressed,sufficient crystallization is difficult to obtain, and desiredcharacteristics are often not attained.

[0036] Stability during mass production is improved by adding Sb₂O₃,which functions as a clarifier. When the composition content is lessthan about 0.1 wt %, there is typically insufficient clarificationeffect, and production characteristics are typically reduced. When thecomposition content exceeds about 9 wt %, crystallization of the glassoften becomes unstable, the deposition crystal phase typically becomesuncontrollable, and the desired characteristics are often difficult toobtain.

[0037] Stability during production is improved by adding As₂O₃, whichfunctions as a clarifier. When the composition content is less thanabout 0.1 wt %, there is often insufficient clarification effect, andproduction characteristics are often reduced. When the compositioncontent exceeds about 9 wt %, crystallization of the glass typicallybecomes unstable, the deposition crystal phase typically becomesuncontrollable, and the desired characteristics are often difficult toobtain.

[0038] The glasses of the present invention may have one or morecrystalline phases and an amorphous phase. The crystalline phasesrepresent about 50 to about 60 percent of the total glass composition.Preferred embodiments include a main crystalline phase of clinoenstatitewhich desirably represents at least about 80 percent by weight of thetotal of all crystalline phases. Preferred embodiments may also includea secondary crystalline phase of, for example, enstatite magnesiumaluminum silicate, and/or zinc titanium oxide which desirably representsless than about 20 percent by weight of the total crystalline phase.

[0039] Although the present invention is described in detail in thefollowing examples, the invention is not limited to these examples.Tables 1-6 show the glass composition in percent-by-weight of Examples1-57. Glass substrates were obtained by the previously describedmanufacturing method in accordance with these numerical examples.

[0040] In the tables, C1 represents a crystal phase of clinoenstatite(MgSiO₃), C2 represents a crystal phase of enstatite (MgSiO₃), M1represents a crystal phase of magnesium aluminum silicate {(Mg Al)SiO₃},Z1 represents a crystal phase of zinc titanium oxide (Zn₂Ti₃O₈) and Z2represents a crystal phase of zinc titanium oxide (Zn₂TiO₄). TABLE 1 Ex.01 Ex. 02 Ex. 03 Ex. 04 Ex. 05 Ex. 06 Ex. 07 Ex. 08 Ex. 09 Ex. 10 SiO₂35.0 35.0 35.0 35.0 35.0 35.0 38.0 38.0 38.0 38.0 Al₂O₃ 13.0 13.5 14.014.5 15.0 15.5 13.0 13.5 14.0 14.5 MgO 28.0 27.5 27.0 26.5 26.0 25.525.0 25.0 25.0 25.0 TiO₂ 18.0 18.0 18.0 18.0 18.0 18.0 18.0 17.5 17.016.5 Li₂O 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 Primary TreatmentTemperature (° C.) 660 660 660 660 660 660 660 660 660 660 SecondaryTreatment Temperature (° C.) 700 700 700 700 700 700 700 700 700 700Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 Secondary TreatmentTemperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa) 137.9 137.3136.6 136 135.3 134.7 142.7 142.7 142.7 142.7 Diameter of Crystal (nm)30 30 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1 C1 C1 C1 C1 C1C1 C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 OtherCrystal Phase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1

[0041] TABLE 2 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Ex. 18Ex. 19 Ex. 20 SiO₂ 38.0 38.0 43.0 43.0 43.0 43.0 43.0 43.0 48.0 48.0Al₂O₃ 15.0 15.5 13.0 13.5 14.0 14.5 15.0 15.5 13.0 13.5 MgO 25.0 25.025.0 23.0 20.0 22.0 20.0 22.0 20.0 22.0 TiO₂ 16.0 15.5 13.0 14.5 17.014.5 18.0 15.5 15.0 12.5 Li₂O 6.0 6.0 6.0 6.0 6.0 6.0 4.0 4.0 4.0 4.0Primary Treatment Temperature (° C.) 660 660 660 660 660 660 660 660 660660 Secondary Treatment Temperature (° C.) 700 700 700 700 700 700 700700 700 700 Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 SecondaryTreatment Temperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa)142.7 142.7 141.5 139.1 135.6 138 135.6 138 148.7 151 Diameter ofCrystal (nm) 30 30 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1C1 C1 C1 C1 C1 C1 C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2C2 Other Crystal Phase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1

[0042] TABLE 3 Ex. 21 Ex. 22 Ex. 23 Ex. 24 Ex. 25 Ex. 26 Ex. 27 Ex. 28Ex. 29 Ex. 30 SiO₂ 48.0 48.0 48.0 48.0 50.0 50.0 50.0 50.0 50.0 50.0Al₂O₃ 14.0 14.5 15.0 15.5 13.0 13.5 14.0 14.5 15.0 15.5 MgO 25.0 25.022.0 20.0 20.0 20.0 20.0 18.0 16.0 15.0 TiO₂ 9.0 8.5 11.0 10.5 11.0 10.510.0 11.5 13.0 13.5 Li₂O 4.0 4.0 4.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 PrimaryTreatment Temperature (° C.) 660 660 660 660 660 660 660 660 660 660Secondary Treatment Temperature (° C.) 700 700 700 700 700 700 700 700700 700 Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 SecondaryTreatment Temperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa)154.5 154.5 151 148.7 153.9 153.9 153.9 151.6 149.2 148.1 Diameter ofCrystal (nm) 30 30 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1C1 C1 C1 C1 C1 C1 C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2C2 Other Crystal Phase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1

[0043] TABLE 4 Ex. 31 Ex. 32 Ex. 33 Ex. 34 Ex. 35 Ex. 36 Ex. 37 Ex. 38Ex. 39 Ex. 40 SiO₂ 43.0 48.0 43.0 48.0 43.0 48.0 43.0 48.0 43.0 48.0Al₂O₃ 15.5 13.0 15.5 13.0 15.5 13.0 15.5 13.0 15.5 13.0 MgO 18.0 16.022.0 17.0 20.0 19.0 20.0 16.0 20.0 15.0 TiO₂ 11.5 6.0 15.0 15.0 15.510.0 15.5 13.0 15.5 15.0 Li₂O 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0ZnO 8.0 13.0 P₂O₅ 0.5 3.0 ZrO₂ 2.0 6.0 CaO 2.0 6.0 Nb₂O₅ 2.0 5.0 PrimaryTreatment Temperature (° C.) 660 660 660 660 660 660 660 660 660 660Secondary Treatment Temperature (° C.) 700 700 700 700 700 700 700 700700 700 Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 SecondaryTreatment Temperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa)133.3 144 138 145.2 135.6 147.5 135.6 144 135.6 142.8 Diameter ofCrystal (nm) 30 30 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1C1 C1 C1 C1 C1 C1 C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2C2 Other Crystal Phase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1 Other Crystal PhaseZ1 Z1 Other Crystal Phase Z2 Z2

[0044] TABLE 5 Ex. 41 Ex. 42 Ex. 43 Ex. 44 Ex. 45 Ex. 46 Ex. 47 Ex. 48Ex. 49 Ex. 50 SiO₂ 43.0 48.0 43.0 48.0 43.0 48.0 43.0 48.0 43.0 48.0Al₂O₃ 15.5 13.0 15.5 13.0 15.5 13.0 15.5 13.0 15.5 13.0 MgO 20.0 15.021.8 16.0 20.0 18.0 22.0 18.0 22.0 20.0 TiO₂ 15.5 15.0 15.5 15.0 15.511.0 15.5 15.0 17.3 16.5 Li₂O 4.0 4.0 4.0 4.0 4.0 4.0 2.0 2.0 2.0 2.0Ta₂O₅ 2.0 5.0 K₂O 0.2 4.0 B₂O₃ 2.0 6.0 Y₂O₃ 2.0 4.0 Sb₂O₃ 0.2 0.5Primary Treatment Temperature (° C.) 660 660 660 660 660 660 660 660 660660 Secondary Treatment Temperature (° C.) 700 700 700 700 700 700 700700 700 700 Primary Treatment Time (hr) 5 5 5 5 5 5 5 5 5 5 SecondaryTreatment Temperature (hr) 5 5 5 5 5 5 5 5 5 5 Young's Modules (G Pa)135.6 142.8 137.7 144 135.6 146.3 138 146.3 138 148.7 Diameter ofCrystal (nm) 30 30 30 30 30 30 30 30 30 30 Main Crystal Phase C1 C1 C1C1 C1 C1 C1 C1 C1 C1 Secondary Crystal Phase C2 C2 C2 C2 C2 C2 C2 C2 C2C2 Other Crystal Phase M1 M1 M1 M1 M1 M1 M1 M1 M1 M1

[0045] TABLE 6 Ex. 51 Ex. 52 Ex. 53 Ex. 54 Ex. 55 Ex. 56 Ex. 57 SiO₂43.0 48.0 43.0 48.0 43.0 48.0 48.0 Al₂O₃ 15.5 13.0 15.5 13.0 15.5 13.013.0 MgO 22.0 18.0 17.0 15.0 20.0 15.0 24.0 TiO₂ 15.5 15.0 15.5 12.015.5 15.0 15.0 Li₂O 2.0 2.0 4.0 4.0 4.0 4.0 Sb₂O₃ 2.0 4.0 5.0 8.0 As₂O₃2.0 5.0 Primary Treatment Temperature (° C.) 660 660 660 660 660 660 660Secondary Treatment Temperature (° C.) 700 700 700 700 700 700 700Primary Treatment Time (hr) 5 5 5 5 5 5 5 Secondary TreatmentTemperature (hr) 5 5 5 5 5 5 5 Young's Modules (G Pa) 138 146.3 132.1142.8 135.6 142.8 153.4 Diameter of Crystal (nm) 30 30 30 30 30 30 30Main Crystal Phase C1 C1 C1 C1 C1 C1 C1 Secondary Crystal Phase C2 C2 C2C2 C2 C2 C2 Other Crystal Phase M1 M1 M1 M1 M1 M1 M1

[0046] The present invention provides a glass substrate having excellentproduction characteristics and a Young's modulus of 110 or higher.

[0047] Although the present invention has been fully described by way ofexamples with reference to the accompanying drawings, it is to be notedthat various changes and modification will be apparent to those skilledin the art. Therefore, unless such changes and modifications depart fromthe scope of the present invention, they should be construed as beingincluded therein.

What is claimed is:
 1. A polished glass disk medium substrate formed ofa mixture of glass forming raw materials comprising about 35% to about50% by weight SiO₂; about 13% to about 15.5% by weight Al₂O₃; about 10%to about 30% by weight MgO; and about 5% to about 20% by weight TiO₂. 2.The polished glass disk medium substrate according to claim 1 , whereinthe raw materials further comprise about 0.1% to about 8% by weightLi₂O.
 3. The polished glass disk medium substrate according to claim 1 ,wherein the raw materials further comprise about 0.1% to about 22% byweight ZnO.
 4. The polished glass disk medium substrate according toclaim 1 , wherein the raw materials further comprise about 0.1% to about5% by weight P₂O₅.
 5. The polished glass disk medium substrate accordingto claim 1 , wherein the raw materials further comprise about 0.1% toabout 12% by weight ZrO₂.
 6. The polished glass disk medium substrateaccording to claim 1 , wherein the raw materials further comprise about0.1% to about 9% by weight CaO.
 7. The polished glass disk mediumsubstrate according to claim 1 , wherein the raw materials furthercomprise about 0.1% to about 9% by weight Nb₂O₅.
 8. The polished glassdisk medium substrate according to claim 1 , wherein the raw materialsfurther comprise about 0.1% to about 9% by weight Ta₂O₅.
 9. The polishedglass disk medium substrate according to claim 1 , wherein the rawmaterials further comprise about 0.1% to about 9% by weight K₂O.
 10. Thepolished glass disk medium substrate according to claim 1 , wherein theraw materials further comprise about 0.1% to about 9% by weight B₂O₃.11. The polished glass disk medium substrate according to claim 1 ,wherein the raw materials further comprise about 0.1% to about 9% byweight Y₂O₃.
 12. The polished glass disk medium substrate according toclaim 1 , wherein the raw materials further comprise about 0.1% to about9% by weight Sb₂O₃.
 13. The polished glass disk medium substrateaccording to claim 1 , wherein the raw materials further comprise about0.1% to about 9% by weight As₂O₃.
 14. The polished glass disk mediumsubstrate according to claim 1 , said raw materials consistingessentially of about 35% to about 50% by weight SiO₂; about 13% to about15.5% by weight Al₂O₃; about 10% to about 30% by weight MgO; about 5% toabout 20% by weight TiO₂.
 15. The polished glass disk medium substrateaccording to claim 14 , further containing one or more of the following:about 0.1% to about 8% by weight Li₂O; about 0.1% to about 22% by weightZnO; about 0.1% to about 5% by weight P₂O₅; about 0.1% to about 12% byweight ZrO₂; about 0.1% to about 9% by weight CaO; about 0.1% to about9% by weight Nb₂O₅; about 0.1% to about 9% by weight Ta₂O₅; about 0.1%to about 9% by weight K₂O; about 0.1% to about 9% by weight B₂O₃; about0.1% to about 9% by weight Y₂O₃; about 0.1% to about 9% by weight Sb₂O₃;and about 0.1% to about 9% by weight As₂O₃.
 16. The polished glass diskmedium substrate according to claim 14 , wherein said substrate isessentially free of BaO, ZrO₂, B₂O₃ and NiO.
 17. The polished glass diskmedium substrate according to claim 1 , comprising crystalline phasesand amorphous phases.
 18. The polished glass disk medium substrateaccording to claim 17 , wherein the crystalline phases represent about50 to about 60 percent by weight of the total glass composition.
 19. Thepolished glass disk medium substrate according to claim 17 , comprisinga crystalline phase of clinoenstatite.
 20. The polished glass diskmedium substrate according to claim 19 , wherein the crystalline phaseof clinoenstatite represents at least about 80 percent by weight of thecrystalline phases.
 21. The polished glass disk medium substrateaccording to claim 17 , comprising a crystalline phase of enstatite. 22.The polished glass disk medium substrate according to claim 22 , whereinthe crystalline phase of enstatite represents less than or equal toabout 20 percent by weight of the crystalline phases.
 23. The polishedglass disk medium substrate according to claim 17 , comprising acrystalline phase of magnesium aluminum silicate.
 24. The polished glassdisk medium substrate according to claim 23 , wherein the crystalinephase of magnesium aluminum silicate represents less than or equal toabout 20 percent by weight of the crystalline phases.
 25. The polishedglass disk medium substrate according to claim 17 , comprising acrystalline phase of Zn₂Ti₃O₈.
 26. The polished glass disk mediumsubstrate according to claim 25 , wherein the crystalline phase ofZn₂Ti₃O₈ represents less than or equal to about 20 percent by weight ofthe crystalline phases.
 27. The polished glass disk medium substrateaccording to claim 17 , comprising a crystalline phase of Zn₂TiO₄. 28.The polished glass disk medium substrate according to claim 27 , whereinthe crystalline phase of Zn₂TiO₄ represents less than or equal to about20 percent by weight of the crystalline phase.
 29. The polished glassdisk medium substrate according to claim 1 , comprising a maincrystalline phase of clinoenstatite and a secondary crystalline phase ofenstatite.
 30. The polished glass disk medium substrate according toclaim 29 , further comprising one or more of a crystalline phase ofmagnesium aluminum silicate, a crystalline phase of zinc titanium oxide,and a crystalline phase of zinc titanium oxide.
 31. The polished glassdisk medium substrate according to claim 1 , wherein said glasssubstrate has a Young's modulus of 110 or higher.
 32. The polished glassdisk medium substrate according to claim 1 , wherein said substrate isprepared by heating glass forming raw materials to a temperature, T₁,between about 500 and 680° C. to generate crystal nuclei; heating at atemperature, T₂, between about 680 and abot 800° C. to grow crystalnuclei; and cooling to obtain crystallized glass.
 33. A recording diskcomprising the polished glass disk medium substrate defined in claim 1 .34. The recording disk according to claim 33 , wherein said recordingdisk is a hard disk.
 35. The recording disk according to claim 33 ,wherein said recording disk is a magnetic disk.
 36. The recording diskaccording to claim 33 , wherein said recording disk is an optical disk.37. The recording disk according to claim 33 , wherein said recordingdisk is a magnetic-optical disk.
 38. A method of making a glass diskmedium substrate comprising: heating glass forming raw materials to atemperature sufficiently high to melt the raw materials; forming a diskmedium substrate; and crystallizing the disk medium substrate, whereinsaid crystallizing comprises heating the disk medium substrate to atemperature, T₁, between about 500 and 680° C. to generate crystalnuclei; heating at a temperature, T₂, between about 680 and about 800°C. to grow crystal nuclei; and cooling to obtain crystalized glass. 39.The method according to claim 38 , further comprising polishing saidglass disk medium substrate.
 40. The method according to claim 38 ,wherein said glass disk medium substrate formed has a Young's modulus of110 or higher.